The present invention relates to a control unit of a disk apparatus, and in particular to a control unit suitable for a small-sized magnetic disk apparatus for implementing high-precision head positioning demanded as the recording density is increased and implementing higher performance of interface function.
In small-sized magnetic disk apparatuses, for example, higher recording density is being promoted by higher performance of interface function, smaller size of apparatus and larger storage capacity.
For attaining this higher recording density, it is important to increase the track density. As for a high-precision head positioning technique corresponding to higher precision of positioning operation associated with increase in track density, there is an embedded servo scheme using a servo signal recorded intermittently into a disk surface for recording of data (data record area), i.e., so-called servo signal embedded in data record area. In addition, there is a hybrid servo scheme as described in U.S. Pat. No. 5,109,307. According to the hybrid servo scheme, both the servo signal embedded in servo face i.e., a disk surface dedicated to recording of a servo signal and a servo signal recorded in data record area are used. During seek operation, seek speed is increased by using the servo signal embedded in servo face which has been continuously written into the servo face.
As a head positioning servo circuit scheme associated with such a servo scheme, a digital servo circuit scheme using a microprocessor is suitable from the viewpoint of flexible application and higher integration of circuit as described in JP-B-4-15548. As for the microprocessor conducting digital servo processing and having a number of quantization steps and a sampling period required for implementing a positioning precision and a response speed which are equal in level to those of a conventional analog servo circuit scheme, there is needed a speed far higher than that of a microprocessor included in the analog servo circuit scheme to conduct servo processing.
Furthermore, with the advance of higher density recording of data in magnetic disk apparatuses, a much higher speed is required of the microprocessor as described hereafter.
As a recording scheme for equalizing linear recording densities in an inner periphery and an outer periphery on the data record area and thereby increasing the area recording density, there is a so-called zone bit recording scheme. In the zone bit recording scheme, the data record area is divided into a plurality of zones in order beginning from the innermost of the data record area and the number of sectors is made larger in an outer periphery zone located outside than in an inner periphery zone located inside as described in U.S. Pat. No. 4,799,112. Small-sized magnetic disk apparatuses adopting this scheme are now increasing. In this scheme, however, the number of sectors changes when high-speed seek operation is conducted over a plurality of zones. As compared with a disk apparatus which does not adopt the zone bit recording scheme, therefore, changeover processing attendant upon the change of the number of sectors with respect to the radial direction of the disk is needed as additional processing. Because of this changeover processing, the microprocessor bears a heavier load.
The speed of disk rotation was typically 3600 revolutions/minute, for example, in the past. In the recent times, however, the waiting time of disk rotation at the time of data access is shortened and the speed of disk rotation for transferring data at high speed is increased. A speed of disk rotation as high as 7200 revolutions/minute is thus demanded.
In this way, high speed is increasingly required of the microprocessor used for head positioning and data read/write control.
On the other hand, there are several kinds as interface between a small-sized magnetic disk apparatus and external devices. As representative interface, there is SCSI (Small Computer System Interface) standardized by ANSI (American National Standard Institute). Even after SCSI which is the first standard, SCSI is subjected to substantial function extension including support commands such as SCSI-2 and then SCSI-3. In response to this, the capacity of the program storing memory needed by an interface control microprocessor of magnetic disk apparatuses has increased.
Among controls exercised by the control unit of the magnetic disk apparatus, principal ones are head positioning control, data read/write control and interface control. As for such architecture that these controls are exercised by a single microprocessor, one described in U.S. Pat. No. 4,819,153 and one described in U.S. Pat. No. 4,979,056 can be mentioned.
Typically in small-sized magnetic disk apparatuses, mechanism for head positioning control and data read/write control is made essentially the same and a plurality of variant models having different interface specifications, i.e., different interface controls for exercising input/output control of data with respect to the outside are developed and produced.
If an apparatus control system for exercising head positioning control employing a digital servo scheme using an embedded servo scheme or a hybrid servo scheme and for exercising data read/write control and an interface function control system relating to input/output control of data with respect to the outside are implemented by a single microprocessor, interface control processing is kept waiting because of digital servo control processing having priority higher than that of interface control processing and being responsive to a processing request generated by a periodic servo signal appearing at least every sector, resulting in a lowered processing capability of interface control.
As a countermeasure against this, there is a method of increasing the speed of processing of the microprocessor, i.e., the speed of the clock in order to make waiting time sufficiently short. As compared with the operation speed in the dual-microprocessor configuration, however, an excessively high speed is demanded in the single-microprocessor configuration. From the viewpoint of circuit technique, the cost of a microprocessor becomes higher as the speed of its operation clock is increased. Therefore, it is economically difficult to use such a microprocessor.
As another countermeasure, there is also a method of developing, for each interface specification, a processing scheduling program with special contrivance applied to a combination of interface control processing and digital servo control processing in order to employ the single-microprocessor configuration restrained in speed increase. This development takes time.
As another solution method, a method of using a plurality of microprocessor packages, i.e., a plurality of microprocessors is conceivable. In a small-sized disk apparatus such as a 2.5 inch size disk apparatus having a disk diameter of 67 mm, however, the circuit board space is approximately 70 mm.times.100 mm. This results in a problem that it is difficult to mount a plurality of microprocessor packages.
A disk drive circuit having product type Z86C95 of Zilog including a CPU and a slave digital signal processor (DSP) in one package is commercially available. However, the digital signal processor is merely used to assist a part of calculation for drive by the CPU.